The present invention relates generally to optical communication systems, and more particularly, to channel selectors useful for wavelength division multiplexed (WDM) optical communication systems.
Optical communication systems typically include a variety of devices (e.g., light sources, photodetectors, switches, optical fibers, modulators, amplifiers, and filters). For example, in the optical communication system 1 shown in FIG. 1, a light source 2 generates optical signals 3. Each optical signal 3 comprises a plurality of wavelengths. The optical signals 3 are transmitted from the light source 2 to a detector 5. Typically, an optical fiber 4 transmits the optical signals 3 from the light source 2 to the detector 5. The optical fiber 4 has amplifiers (not shown) and filters (not shown) positioned along its length. The amplifiers and filters propagate the optical signals 3 along the length of the optical fiber 4 from the light source 2 to the detector 5.
Some optical communications systems include a single channel. Single channel optical communications systems transmit optical signals at a single specified channel wavelength. The transmission rate of a single channel is about 2.5 Gbits/sec (Gigabits/second). Consequently, the transmission capacity of single channel optical communication systems is limited to about 2.5 Gbits/sec.
One technique used to increase the transmission capacity of optical communication systems is wavelength division multiplexing (WDM). WDM optical communication systems are multi-channel systems. Each channel of the multi-channel system transmits optical signals using wavelengths that are different from one another. In WDM systems, optical signals are simultaneously transmitted over the multiple channels.
WDM systems are desirable because their total transmission capacity increases relative to single channel systems as a function of the number of channels provided. For example, a four-channel WDM optical communication system can transmit optical signals at a rate of about 10 Gbit/sec (4xc3x972.5 Gbit/sec). Since a single channel system only transmits optical signals at a rate of about 2.5 Gbit/sec, the four-channel WDM system has about 400% greater capacity than the single-channel system.
In a WDM optical communication system, the optical signals are multiplexed over multiple channels. Thereafter, the multiplexed optical signals are transmitted over a waveguide (e.g., optical fiber). At the receiving end, the multiplexed optical signals are demultiplexed such that each of the multiple channels is routed individually to a designated receiver by a channel selector. Typically, the channels are routed using mode couplers (e.g., multi-mode interference couplers or star couplers) or diffraction gratings.
In order for the channel selector to route each channel to the designated receiver, it is desirable that the wavelengths of the selected channels correspond to the passband of the channel selector. The term passband as used in this disclosure refers to the band of wavelengths transmitted by the channel selector, denoted as 10 in FIG. 2A. When the wavelengths of the selected channels and the passband of the channel selector do not correspond, the channel selector can not route the selected channels to the designated receivers. Additionally, it is desirable for the channel selector to have a passband with sharp cut-off regions. The cut-off regions of the passband refer to the transition region from the passband to the stopband, denoted as 12 in FIG. 2A. The term stopband as used in this disclosure refers to the band of wavelengths not transmitted by the channel selector, denoted as 11 in FIG. 2A. Sharp cut-off regions have steep slopes. When the passband of the channel selector does not have cut-off regions with steep slopes adjacent channels potentially interfere (crosstalk) with each other. Interference between adjacent channels, denoted as 14 in FIG. 2B, causes transmission errors.
Oda, K. et al., xe2x80x9cA Wide-Band Guided-Wave Periodic Multi/Demultiplexer with a Ring Resonator for Optical FDM Transmission Systemsxe2x80x9d, IEEE J. Light. Tech., Vol. 6, No. 6, pp. 1016-1022 (1988) describes a channel selector for use in a WDM optical communication system. The channel selector has a structure which combines a single Mach-Zehnder Interferometer (MZI) with a ring resonator. In Oda et al., the coupling ratio between the ring resonator and the MZI has a fixed value of 8/9. This fixed value for the coupling ratio limits the passband width of the Oda et al. channel selector to about half the free spectral range (FSR). Typically, the FSR describes the band of multiplexed channel wavelengths transmitted in a WDM system. Since the passband width of the channel selector only corresponds to half the wavelengths of the FSR, the Oda et al. channel selector is useful for selecting only a limited number of multiplexed channels.
Another channel selector for a WDM optical communication system, an autoregressive moving average (ARMA) filter, is described in Jinguji, K., xe2x80x9cSynthesis of Coherent Two-Port Optical Delay-Line Circuit with Ring Waveguidesxe2x80x9d, IEEE J. Light. Tech., Vol. 14, No. 8, pp. 1882-1898 (1996). Autoregressive moving average filters include both poles and zeros in their transfer functions. The Jinguji ARMA filter includes a structure that has a cascade of MZIs with a single ring resonator on one arm of each MZI. Both the MZIs and the ring resonators also include phase shift controllers. Such a filter architecture is complex and difficult to fabricate since a large number of MZIs are needed for channel selection. Additionally, all Jinguji filter architectures must be ring resonator based structures.
Thus, channel selectors for use in WDM systems that have advantages over prior art channel selectors are sought.
The present invention is directed to a channel selector useful for a multi-channel WDM optical communications system. The channel selector selects one or more channels from input multi-channel optical signals for routing to predetermined destinations within the optical communication system. Multi-channel optical signals include two or more channels, wherein each channel transmits optical signals of a specified wavelength. The channel selector selects one or more channels by first splitting (dividing) portions of the input multi-channel optical signals across two or more optical paths. Thereafter, a desired phase response is applied to the split portions of the multi-channel optical signals. The desired phase response is applied to the split portions of the multi-channel optical signals so the channels therein interfere with each other either constructively or destructively when the split portions of the multi-channel optical signals are combined.
The desired phase response is applied to the split portions of the multi-channel optical signals by shifting the phase of each channel in the multi-channel optical signal as a function of frequency. Phase-shifted channels which have approximately the same phase constructively interfere with each other when the sum of their magnitudes is greater than zero. Phase-shifted channels destructively interfere with each other when the sum of their magnitudes approximates zero. Channels which interfere with each other constructively are selected by the channel selector. Channels which interfere with each other destructively are not selected by the channel selector.
The channel selector of the present invention has a structure which includes a plurality of input ports, a plurality of output ports, a splitter, a combiner, one or more all-pass optical filters, and a plurality of optical paths. Each optical path has one input and one output. The plurality of input ports are coupled to the inputs of the plurality of optical paths via the splitter. The outputs of the plurality of optical paths are coupled to the plurality of output ports via the combiner. The one or more all-pass optical filters are coupled to the plurality of optical paths.
The splitter determines what portion of input multi-channel optical signals are split (divided) across the plurality of optical paths. Typically, the multi-channel optical signals are split across the plurality of optical paths with couplers (e.g., multi-mode interference couplers and star couplers).
After the multi-channel optical signals are split across the plurality of optical paths, the one or more all-pass optical filters apply the frequency dependent phase shift (e.g., time delay) to each channel of the multi-channel optical signals provided thereto. Thereafter, the combiner combines the phase shifted multi-channel optical signals, directing selected channels to predetermined destinations within the optical communication system through one or more of the plurality of output ports. Typically, the multi-channel optical signals are combined using couplers (e.g., multi-mode interference couplers and star couplers).
Each of the one or more all-pass optical filters of the channel selector of the present invention includes at least one feedback path, a splitter/combiner, a filter input port, and a filter output port. The splitter/combiner is coupled to at least one of the feedback paths, the filter input port, and the filter output port.
Coupling ratios for the splitter/combiner and the feedback path determine what portions of the multi-channel optical signals are coupled into and away from the feedback path from the optical path. The magnitude of the coupling ratios for the splitter/combiner and the feedback path are a matter of design choice.
The at least one feedback path of the all-pass optical filter applies the desired phase response to each channel of the multi-channel optical signals transmitted therethrough. Each of the at least one feedback paths of the one or more all-pass optical filters has a path length. The path lengths of each feedback path are optionally different. Feedback paths with different path lengths are desirable because they potentially increase the FSR of the channel selector.
In one embodiment of the present invention, the at least one feedback path of the one or more all-pass optical filters has a ring resonator structure. The ring resonator structure includes one or more ring resonators where each of the ring resonators is a closed loop. The one or more ring resonators are optionally arranged as a ring cascade or as a series of coupled rings. For the ring cascade, one ring resonator is coupled to the splitter/combiner with the remaining ring resonators, coupled one to another. In the series of coupled rings, each ring is coupled to the splitter/combiner.
In an alternate embodiment of the present invention, the at least one feedback path of the all-pass optical filter includes a cavity and a plurality of reflectors. At least one reflector of the plurality of reflectors has a reflectivity of about 100%, while the remaining reflectors are partial reflectors with reflectivities less than 100%. Twice the length of the cavity approximates the path length of the feedback path. The partial reflectors perform the functions of the splitter/combiner.
Another embodiment of the present invention uses a photonic band gap (PBG) structure as the at least one feedback path of the one or more all-pass optical filters. The photonic band gap (PBG) structure includes periodic layers of a material which confine a range of wavelengths within such periodic layers. Defects formed in a 2-dimensional array of such layers (2-D PBG) provides a guided feedback path for multi-channel optical signals propagated therein. Point defects optionally formed at the edges of the 2-D PBG structure perform the functions of the splitter/combiner, coupling optical signals into and away from such feedback path.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and do not serve to limit the invention, for which reference should be made to the appended claims.